The tubular portions are sealed at one end to make a parison. For this the end portion of the tube is heated to welding and forming temperature. A hemi-spherical mandrel and a bottom mold are used to seal the heated end portion of the tube in a forming and welding operation (German Pat. No. 1 704 119).
In a similar operation the opposite end portion is formed to a finish portion in using a mandrel and an outer mold (German publication No. 3 101 284). U.S. Pat. No. 3,929,943 discloses to provide the end of a tubular portion with a circumferential bevel to enhance sealing and welding the end portion.
A single amorphous thermoplastic material, mainly polyethylene terephthalate (PET) may be used for the parisons. The hollow articles which are blowmolded under conditions of biaxial orientation exhibit excellent mechanical features and good barrier characteristics with respect to oxygen and carbonic acid.
However, there is a number of liquids which need an improved gas barrier feature of the hollow article. To obtain this improvement the hollow article is made of a multilayered parison, wherein the inner and outer layers are made of a thermoplastic material, for example PET and the intermediate layer or, respectively intermediate layers are made of a material having a high barrier characteristic.
Unfortunately, there is the danger, that the intermediate layer barrier prevents an intimate welding of the inner and outer layer when a multilayered tubular structure is heat formed in its end portions. In particular, the material of the medium layer flows towards the welding center in the sealing process, thus preventing proper welding of the inner to the outer layer. At best a weakened welding zone results.